The present invention relates generally to antiskid control system, and more specifically to an antiskid control system which detects an overshooting hydraulic brake pressure on vehicle wheels immediately following the application of brake to optimize the hydraulic brake action of the vehicle.
When the depression of a brake pedal is increased to apply a higher hydraulic pressure to spinning wheels, the latter tends to be locked and slips on the road surface. If the locking occurs in the front wheels, the traction is lost and the steering is out of control, and if it occurs in the rear wheels the lateral force is lost, causing the tail of the vehicle to oscillate sideways. Rapid acceleration will therefore bring about a very dangerous situation. Various anti-lock control systems have been in use to prevent the dangerous situation.
In an antiskid control system, an actuator, or pressure modulator modulates the hydraulic pressure in a relief, boost or hold mode in accordance with sensed operating parameters. Various methods have been proposed for the antiskid systems, which include:
(1) In one prior antiskid control system the rate of slippage is derived from wheel speed and hypothetical vehicle speed and the hydraulic pressure is controlled such that the slip rate is maintained within a prescribed range.
(2) According to a second prior art system, the acceleration and deceleration of a vehicle wheel is detected and compared with a higher threshold to determine that a wheel lock condition has occurred if the higher threshold is exceeded. Hydraulic pressure is modulated so that the wheel acceleration (or deceleration) drops below a lower threshold which indicates that wheel acceleration is restoring to the normal.
(3) A third prior art system combines the features of the first and second prior art systems as shown and described in Japanese Examined Patent Publication No. 59-20508.
However, the prior antiskid control systems involve a substantial amount of first-order time delays associated with the determination of wheel speed and acceleration, hypothetical vehicle speed and slip rate, and the time taken to operate the actuator, and the time taken for the fluid under pressure to change to calculated value. Further involved are delay factors associated with filters for eliminating undesirable ripples introduced to the system through wheel speed detectors and those associated with the inertia of the wheels. Due to the various time delay factors, the prior antiskid control system using the slip rate and wheel acceleration as control parameters tends to produce excessive pressure reduction control. Specifically, hydraulic pressure relief action tends to extend beyond the calculated period, causing an excessive bleeding of pressurized fluid. This results in a low braking efficiency and causes the bleeding fluid to produce a large kickback effect on the brake pedal as it returns to the master cylinder and eventually leads to a violent fluctuation of braking force, which can cause vibration of the steering wheel.
To overcome this problem, one prior system as shown and described in Japanese Provisional Patent Specification No. 59-130767 employs a method whereby a pressure relief action is performed for a predetermined period of time followed by a period of constant pressure. However, this system is still unsatisfactory because of the constantly changing friction coefficient of the wheels depending on varying road surfaces and because of the driver's braking action tending to vary erratically from time to time.